This invention relates to the fabrication of opaque glass-ceramic articles exhibiting an integral beige tint which are eminently useful as culinary ware.
As is well-recognized in the art, glass-ceramic articles are produced through the controlled crystallization of precursor glass articles, the process of manufacture normally consisting of three basic steps: first, a glass forming batch typically containing a nucleating agent is melted; second, that melt is cooled to a temperature below the transformation range thereof and simultaneously shaped into a glass body of a desired configuration; and third, that glass body is exposed to a heat treatment designed to effect the in situ growth of crystals within the glass body. (As is commonly used in the art, the transformation range is defined as the temperature at which a molten material is transformed into an amorphous mass, that temperature being deemed to reside in the vicinity of the annealing point of a glass.)
Quite frequently, the thermally-induced crystallization in situ will be carried out in two general steps: first, the precursor glass body will be heated to a temperature slightly above the transformation range for a period of time sufficient to generate nuclei therein; and second, the nucleated glass is heated to a temperature approaching, and often surpassing, the softening point of the glass to cause the growth of crystals on the nuclei. This two-stage heat treatment commonly yields glass-ceramic articles containing higher levels of crystallization with more uniformly-sized, fine-grained crystals. It will be appreciated that, as the temperature of the nucleated precursor glass article approaches the softening point of the glass, the rate at which the temperature is raised must be regulated to allow time for the sufficient growth of crystallization to resist thermal deformation of the body. Thus, the crystals developed during the heat treatment process are most usually more refractory than the precursor glass, and thereby can provide a structure demonstrating resistance to thermal deformation at temperatures higher than those at which the precursor glass can be subjected. Also, because the crystal forming components will have been removed therefrom, the small percentage of residual glass remaining in the glass-ceramic (customarily less than 50% by volume and frequently less than 10% by volume) will have a very different composition from that of the precursor glass, and most often that residual glass will manifest a higher softening point than that of the precursor glass.
The development of a high concentration of crystals within a glass-ceramic body has a further advantage vis-a-vis the precursor glass body in dramatically enhancing the mechanical strength thereof, usually by a factor of at least two and frequently as much as three times that of the precursor glass. That significant improvement in mechanical strength, coupled with their substantially higher use temperatures and their intrinsic "porcelain-like" appearance, have led to the widespread use of glass-ceramic articles as culinary ware.
Generally, in the absence of added colorants, opaque glass-ceramic articles display a white appearance. For example, Corning Code 9608 glass-ceramic, marketed by Corning Incorporated, Corning, N.Y. for over 30 years under the trademark CORNING WARE.RTM. , exhibits a creamy white appearance. Having a composition included within U.S. Pat. No. 3,157,522, that opaque glass-ceramic contains a crystallinity in excess of 90% by volume wherein betaspodumene solid solution constitutes the predominant crystal phase with a minor amount of spinel and rutile also being present. Corning Code 9608 has the following approximate analysis, expressed in terms of weight percent on the oxide basis:
______________________________________ SiO.sub.2 69.5 ZnO 1.0 F 0.03 Al.sub.2 O.sub.3 17.7 TiO.sub.2 4.7 Fe.sub.2 O.sub.3 0.05 Li.sub.2 O 2.7 ZrO.sub.2 0.2 B.sub.2 O.sub.3 0.07 MgO 2.6 As.sub.2 O.sub.3 0.6 MnO.sub.2 0.03 ______________________________________
As might well be expected, colorants known in the glass art have been incorporated into precursor glass compositions which have subsequently been crystallized in situ to glass-ceramic articles. U.S. Pat. No. 4,461,839 (Rittler) and U.S. Pat. No. 4,786,617 (Andrieu et al.) are recent illustrations of that practice.
The former patent discloses the manufacture of opaque glass-ceramic articles containing .beta.-spodumene solid solution as the predominant crystal phase, which can display colors ranging from gray to brown to almond to beige to yellow to blue, that are prepared from precursor glass articles having base compositions essentially free from MgO and consist essentially, in weight percent, of:
______________________________________ SiO.sub.2 63.5-69 BaO 0-5 Al.sub.2 O.sub.3 15-25 TiO.sub.2 2-3 Li.sub.2 O 2.5-4 ZrO.sub.2 0.5-2.5 Na.sub.2 O 0.1-0.6 As.sub.2 O.sub.3 0.4-0.8 K.sub.2 O 0.1-0.6 Fe.sub.2 O.sub.3 0.05-0.1 ZnO 0-2 ______________________________________
The desired colors are obtained through the use of a "color package" containing about 0.5-3% TiO.sub.2 and up to 0.15% Fe.sub.2 O.sub.3 with 0.3-3% total of at least two oxides in the indicated proportion selected from the group of up to 0.3% V.sub.2 O.sub.5, up to 3% CeO.sub.2, up to 2% CaO, up to 1% NiO, up to 1% WO.sub.3, and up to 1.5% SnO.sub.2. The total TiO.sub.2 content in the glass will range &gt; 2.5-6% and that of the Fe.sub.2 O.sub.3 content will range 0.05-0.2%.
The latter patent describes the fabrication of opaque glass-ceramic articles containing potassium fluorrichterite and/or a related fluormica as the predominant crystal phase(s) from precursor glass compositions essentially free from Li.sub.2 O and which consist essentially, in weight percent, of:
______________________________________ SiO.sub.2 61-70 K.sub.2 O 2.5-5.5 Al.sub.2 O.sub.3 2.75-7 Na.sub.2 O + K.sub.2 O &lt;6.8 MgO 11-16 F 2-3.25 CaO 4.75-9 BaO 0-3.5 Na.sub.2 O 0.5-3 P.sub.2 O.sub.5 0-2.5 ______________________________________
The text of the patent noted that it was possible to incorporate such conventional glass colorants as Fe.sub.2 O.sub.3, CeO.sub.2, CaO, Cr.sub.2 O.sub.3, CuO, MnO.sub.2, Na.sub.2 O, and V.sub.2 O.sub.5 into the base precursor glass composition in amounts typically less than 1% total. Nevertheless, only the use of Fe.sub.2 O.sub.3 to impart a yellow tint to the glass-ceramic was expressly mentioned.
Corning Incorporated currently markets a line of opal glass tableware under the trademark CORNERSTONE.RTM. . That product has a composition included in U.S. Pat. No. 4,331,769 (Danielson et al.) and exhibits a beige tint defined within the polygon bounded by Points ABCDEFA depicted in the appended drawing, which polygon encompasses a plot of the x and y chromaticity coordinates (Illuminant C). The visual appearance of the product is described here in accordance with the standard CIE system utilizing chromaticity coordinates x and y and the tristimulus value Y. Thus, the values are measured under standard conditions, i.e., Illuminant C, with a Hunter Colorimeter and represent the light that diffusely reflects off opaque surfaces. Because the values obtained are readily reproducible, they are commonly employed to facilitate comparisons and to delimit specifications.
The tint is imparted to the CORNERSTONE.RTM. tableware through the incorporation of NiO into the base glass compositions. The research leading to the present invention had as its goal the development of an opaque glass-ceramic body demonstrating properties suitable for use as cookware which would exhibit a hue close to and compatible with that of CORNERSTONE.RTM. tableware, thereby offering to the consumer market a complete line of dinnerware and cookware of approximately the same tint. Because the glass-ceramics were destined for use as culinary ware, the chemical and physical properties recognized in the art as being necessary in such articles would likewise be required in the tinted articles. For example, the tinted articles would exhibit low linear coefficients of thermal expansion, viz., &lt; 15 and preferably &lt; 13.times.10.sup.-7 /.degree.C. over the temperature range of 0.degree.-300.degree.C., and good resistance to the chemical attack of food products.